Site key are unique site identifier which is necessary for crediting publisher account and ads targeting. It is a alphanumeric string and contains 7 to 30 characters.
Check your site keys on My Sites page.
The new key format is being used because it is shorter and easier to transmit. It is completely compliant with previous generations and an old key could be converted to the new generation by cutting off everything past the first 7 characters. Both versions are equally supported.
44fe67f is a second generation version of